The invention relates to pumps and has particular relationship to pumps for driving or transmitting liquid metal. Typically the pumps with which this invention concerns itself serve to circulate liquid metal coolant through a nuclear reactor. The liquid may be sodium at a temperature at the cool leg of the metal circuit of about 400.degree. C. It is contemplated that two pumps are provided, a primary pump within the reactor vessel and a secondary pump outside of the reactor vessel where the temperature is high. Typically the liquid is supplied by a pump at the rate of 120,000 gallons per minute. The pumps constitute a major physical and economic factor in a power plant including a nuclear reactor.
In accordance with the teachings of the prior art, the liquid is circulated by centrifugal pumps. AC induction (electromagnetic) pumps have also been proposed for this purpose. Centrifugal and AC induction pumps are excessively large and costly and AC induction pumps are inefficient in use.
An alternative to the centrifugal and AC induction pumps is the DC electromagnetic pump. Typically the liquid is advanced by the interaction of direct current flowing through the liquid with a constant magnetic field at right angles to the direction of the direct current and the flow of the liquid. In the past, low efficiencies, between 20% and 40%, have been experienced in the use of the DC electromagnetic pumps and their use has been limited. Induction pumps may achieve an efficiency of about 40%. Conventional mechanical (centrifugal) pumps may achieve an efficiency as high as 70% but these pumps are not as reliable as the electromagnetic pumps.
The liquid metal in a DC electromagnetic pump is conducted through a duct. The electrodes through which the current is supplied to the metal constitute spaced walls of the duct. If the wall sections interconnecting the electrodes are of material having substantial electrical conductivity, the shunt current which flows through the wall sections contributes materially to the inefficiency of the DC electromagnetic pump. It has been proposed that the interconnecting wall sections be made of electrically insulating material. (See Kidwell U.S. Pat. No. 3,219,851) However the pressure of the liquid within the ducts is very high, at least 100 pounds per square inch. To withstand the pressure, the insulating material must have good high strength properties and resist corrosion and erosion. A typical material which meets those requirements is high density aluminum oxide, which can only be used structurally in compression.
The current which is conducted through the liquid is very high; typically 500,000 to 1,000,000 amperes. The losses in the generator which supply the current are another important factor contributing to inefficiency. A further contributing factor is the loss in the electromagnet which impresses the field.
It is accordingly an object of this invention to overcome the disadvantages and drawbacks of the prior art and to provide a direct current electromagnetic pump for transmitting a driving liquid metal which shall operate efficiently. It is also an object of this invention to provide such an electromagnetic pump in whose operation the losses resulting from the supply of current to the liquid metal and the losses in the electromagnet shall be minimized.